1. Field of the Invention
This invention relates to the magnetic gearing of permanent magnet brushless motors.
Permanent magnet brushless motors are known which are capable of providing variable speed outputs. The motor characteristics are linear, generating high torque at low speeds and high speed at low torque levels.
In certain applications, the range of speed and torque characteristics of a particular motor may not be sufficient to cover the desired range, even though the output power of the motor may be sufficient. In such circumstances two options are available. Firstly, a more powerful motor could be used to cover the entire range or secondly, mechanical gears could be provided for the motor. Both of these methods add cost and weight to the system.
2. Related Background Art
Canadian Patent Application No. 2341095 discloses an alternative to the above-mentioned methods which uses a technique in which the speed and torque can be varied inside the motor and the only additional item required is a switching circuit. A prerequisite of this technique is that the stator coils of the motor must be segmented into at least two or more sections, which are evenly or perhaps unevenly distributed throughout the stator slots. The switching circuit can then be used to change the number of coil segments which are connected to the supply. Such an arrangement utilises the control of the induced back electromotive force (back emf) to control the speed by selectively altering the number of conductors which are connected to the supply. This in effect also alters the torque with changing speed of the motor.
In the main embodiment of Canadian Patent Application No. 2341095, each of the motor windings comprises a plurality of series-connected sections provided by tappings in the winding, which can be selectively connected across the supply. With just one of the coil segments connected across the supply, the motor will produce a high speed but a low torque. However, with a higher proportion of coils connected in series across the supply, the motor will produce a lower speed at the same torque. In this manner, the speed but not the torque of the motor can be varied by selectively connecting the windings in series.
In an alternative embodiment, each of the motor windings comprises a plurality of parallel-connected sections, which sections can be selectively connected in parallel across the supply. With just one of the coil segments connected across the supply, the motor will produce a high speed but a low torque as previously described. However, with a higher proportion of coils connected in parallel across the supply, the motor will produce high torque at the same speed. In this manner, the torque but not the speed of the motor can be varied by selectively connecting the windings in parallel.
A disadvantage of either arrangement is that sections are redundant when running the motor during some configurations and thus copper (I2R) losses will be higher because the cross-sectional area of copper utilised decreases as the number of active sections decreases. Also, the presence of redundant sections means that the net resistance of the coils is not optimised in all configurations and hence the supply current or voltage has to be controlled to avoid damaging the connected coils. Since speed and torque are functions of the current, any limitation of the current affects the performance of the motor.
In most situations, the supply current to the motor is limited (for example in domestic mains to 13 amps), and thus the attainable speed and torque will not be optimised when some coils are out of circuit.
We have now devised a permanent magnet brushless motor which alleviates the above-mentioned problem.